2,3,5-Trichloropyridine is useful as an intermediate compound for the preparation of various pesticides for the control of mites, insects, bacteria and fungal organisms. Several processes have been used in the past for the preparation of 2,3,5-trichloropyridine.
U.S. Pat. No. 4,108,856 discloses a method for preparing 2.3.5-trichloropyridine by the vapor phase chlorination of 3,5-dichloropyridine at a temperature in the range from 300.degree. C. to 460.degree. C. 2,3,5,6-tetrachloropyridine is also produced during that reaction. The amount of 2,3,5,6-tetrachloropyridine, however, is appreciably smaller than the amount of 2,3,5-trichloropyridine product.
U.S. Pat. No. 4,127,575 discloses a process for preparing 2,3,5-trichloropyridine by the oxidation of 2,3,5-trichloro-6-hydrazinopyridine with or without 2,3,5-trichloro-4-hydrazinopyridine by an alkaline hypochlorite in the presence of a reaction medium at temperatures in the range of from room temperature to the boiling point of the reaction mixture. U.S. Pat. No. 4,379,938 discloses a method for preparing 2,3,5-trichloropyridine by reacting 3,5-dichloro-2-pyridone in a solvent with phosgene in the presence of an N,N-disubstituted formamide at a temperature in the range from 30.degree. C. to 105.degree. C.
2,3,5,6 or symmetrical tetrachloropyridine is used for the preparation of herbicides and pesticides. Numerous processes have been employed in the past for the preparation of 2,3,5,6-tetrachloropyridine.
Some processes utilize the chlorination of other chloropyridines for the production of 2,3,5,6-tetrachloropyridine. U.S. Pat. No. 3,186,994 discloses a method for preparing 2,3,5,6-tetrachloropyridine by reacting chlorine and 3,5-dichloro-2-(trichloromethyl)pyridine or 2,3,5-trichloro-6-(trichloromethyl)pyridine at temperatures of at least 160.degree. C. U.S. Pat. Nos. 3,538,100 and 4,281,135 disclose processes for the preparation of 2,3,5,6-tetrachloropyridine by reacting 2,6-dichloropyridine with chlorine in the presence of a catalyst. In U.S. Pat. No. 4,256,894, 2,3,5,6-tetrachloropyridine is produced by reacting chlorine with 2-chloro-6-(trichloromethyl)pyridine in the presence of a catalyst at temperatures ranging from 160.degree. C. to 220.degree. C. and pressures ranging from 15 psig to 220 psig.
Other processes for the production of 2,3,5,6-tetrachloropyridine disclosed in the past utilize pentachloropyridine as one of the reactants. U.S. Pat. No. 3,993,654 discloses a process for preparing 2,3,5,6-tetrachloropyridine by heating pentachloropyridine in an aqueous medium under pressure, with agitation, in the presence of zinc and hydrogen chloride. U.S. Pat. No. 4,259,495 discloses a process for the preparation of 2,3,5,6-tetrachloropyridine by dechlorinating pentachloropyridine in an ester solvent in the presence of an ammonium salt of an inorganic or an organic acid. U.S. Pat. No. 4,321,389 discloses a process for the production of 2,3,5,6-tetrachloropyridine by transhalogenating pentachloropyridine by a bromide salt in a polar, aprotic solvent at a temperature from about 100.degree. C. to about 140.degree. C. followed by selectively debrominating by hydrogen in the presence of a noble metal catalyst and an acid acceptor. Pentachloropyridine is also used as a reactant in a process disclosed in U.S. Pat. No. 4,322,538 for producing 2,3,5,6-tetrachloropyridine by reacting the pentachloropyridine with iodide ions and a proton donor in a polar, aprotic solvent at a temperature from about 100.degree. C. to 200.degree. C.
Other processes for the production of 2,3,5,6-tetrachloropyridine are disclosed in U.S. Pat. No. 4,127,575 wherein 2,3,5,6-tetrachloropyridine is produced by reacting tetrachloro-4-hydrazinopyridine with alkaline hypochlorite in a reaction medium at temperatures from room temperature to the boiling point of the reaction mixture, and in U.S. Pat. No. 4,327,216 wherein 2,3,5,6-tetrachloropyridine is produced by reacting trichloroacetyl chloride with acrylonitrile in an inert organic solvent in the presence of a catalyst.
The prior art discloses only one method for the production of appreciable amounts of 2,3,5-trichloropyridine and 2,3,5,6-tetrachloropyridine from 3,5-dichloropyridine, a by-product of reactions carried out for the preparation of other chlorinated pyridines. That method is the vapor phase chlorination disclosed in the aforementioned U.S. Pat. No. 4,108,856. One disadvantage of that method is that it is carried out at elevated temperatures in the range from 300.degree. C. to 400.degree. C. and that it involves the handling of gaseous materials. Another disadvantage is that it appears to favor the formation of 2,3,5-trichloropyridine over 2,3,5,6-tetrachloropyridine.
While past attempts to chlorinate 3,5-dichloropyridine in the liquid phase resulted in a chloropyridine yield strongly favoring the formation in undesirable 2,3,4,5 or unsymmetrical tetrachloropyridine, the present invention discloses a novel and economical process for the preparation of 2,3,5-trichloropyridine and/or 2,3,5,6-tetrachloropyridine from 3,5-dichloropyridine that is carried out at a low temperature with the chlorinated pyridine reactant and products being in the liquid phase. Furthermore, the present invention discloses a novel process that favors the formation of 2,3,5,6-tetrachloropyridine over 2,3,5-trichloropyridine.
These and other advantages of the present invention will become apparent from the following description.